The present invention relates to electrical circuits and more particularly to a system and method for adjusting common mode voltage ranges on a differential input device.
A functional differential input device, such as a data communication receiver, an operational amplifier or a comparator, has a differential input that can accommodate only a particular range of common mode voltage signal. However, the actual common mode signal range presented to the device could be quite wide due to several external uncontrollable factors. Conventional data communication transceivers use a differential comparator input circuit to sense differential signals overlapped on a common mode signal. Typically, the common mode signal level required for reliable operation of the input stage of the functional device needs to be such that the input network is within a required common mode voltage range. The differential signal level presented to the input network needs to be substantially greater than the differential noise/error signal level.
One particular conventional device for maintaining a particular common mode voltage range utilizes a prototypical input stage differential transistor pair tied to the common mode signal input through a resistor network. The resistor network senses the difference in voltage between the applied voltage and a reference voltage. The difference in voltage is then translated to an input comparator based on the ratio of resistor divider pairs in the resistor network. The common mode signal at the inputs is adjusted by the resistor divider pairs of the resistor network. Essentially, the resistor divider pairs translate the common mode signal to a value that can be accommodated by the functional circuit. While widely applicable and well suited for many applications, the resistor common mode conversion has some drawbacks. For example, the differential signal is attenuated along with the common mode signal to a low signal level. When used for common mode applications much beyond the supply voltages, the differential signal suffers substantial attenuation due to high resistor ratio requirements on the resistor divider pairs. The high resistor divide ratio is unavoidable due to the requirement of wide common mode signal translation.
Some receiver types or interface circuits are designed to meet one of several interface standards. The Electronics Industry Associates (EIA) has developed several standards to simplify the interface in data communication application. One standard type method is single ended type, while another standard type method is differential which uses two signal lines. Differential data transmission standards are often the choice for transmitting over long distances and through noisy environments. These types of data communication applications require that receivers handle wide variations in common mode voltages. For example, the RS-442 (TIA/EIA-422-B, adopted in 1994) and the RS-485 (EIA RS-485, adopted in 1994) standard for data communication requires that the receiver or interface circuit be capable of handling wide common mode swings in the xe2x88x927V to 12V range.
However, for industrial applications that require wide tolerances on bus voltage variation, the receiver circuitry needs to be capable of handling common mode variations that are substantially higher than the xe2x88x927V to 12V range. Existing design techniques for RS-442 and RS-485 devices are not equipped to handle such wide common mode variations. Several issues like differential signal attenuation and offset/mismatch errors exist with the current topologies which limits the ability to accommodate wide common mode variations.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention provides for a system and methodology to translate a wide common mode voltage range into a narrow common mode voltage range. The present invention accomplishes this task by detecting and adjusting the wide common mode voltage range into a narrow common mode voltage range with minimal loss of differential voltage signal. A feedback technique is employed to extend the common mode voltage range of existing differential devices, such as a receiver, an operational amplifier, a comparator or other functional device operable to receive a differential input signal and a common mode input signal. The wide common mode voltage level is translated to a narrow common mode voltage level that can be accommodated by the differential input of a functional circuit. The technique minimizes the differential signal loss while achieving the above mentioned common mode signal translation. The system and method provide for a fast common mode translation system that can be employed for use with a variety of differential devices.
In one aspect of the invention, an attenuator network, a common mode voltage sensing device and a negative feedback system are employed to provide a system for translating a wide common mode voltage signal into a narrow common mode voltage signal. The common mode voltage sensing device detects the common mode voltage on the differential input of a functional circuit. The common mode voltage sensing device provides an output that is a reference voltage or an output that is an error signal that modifies or dynamically adjusts the reference voltage depending on the particular implementation being employed. The output of the common mode voltage sensing apparatus is then fed to a feedback amplifier stage. The amplifier stage sets a reference control voltage based on a reference voltage that is set, adjusted and controlled by the common mode voltage sensing device. The reference control voltage along with the voltage attenuator network modifies the applied wide common mode voltage to a required narrow common mode voltage.
The voltage sensing apparatus can be employed using resistor devices, such as a resistor divider structure. Alternatively, high impedance Metal-Oxide-Semiconductors (MOS) devices are employed to provide a voltage sensing apparatus. One particular MOS device can be a current comparator that converts the common mode voltage signal to a current. The current is then compared with a reference current to provide a current error signal (e.g., IERROR) relative to the common mode input signal. Another MOS device can be a pair of inverter circuits coupled to one another and the input of the attenuation network. The inverter circuits can employ current sources to generate a current error signal relative to the common mode input signal.
The following description and the annexed drawings set forth certain illustrative aspects of the invention. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.